The present invention relates to a process for preparing water-soluble polymers having a stickiness, and more particularly to a process for preparing sticky polymers in a good efficiency with a remarkably decreased adhesion to a polymerization apparatus.
Various water-soluble polymers have been produced from vinyl group-containing monomers e.g. acrylamide or methacrylamide: acrylic or methacrylic acid salts: nitrogen-containing acrylic acid esters such as quaternary salts or salts with acids of N,N-dialkylaminoalkyl acrlyates or methacrylates and quaternary salts or salts with acids of N,N-dialkylaminohydroxyalkyl acrylates or methacrylates: diallylamine compounds such as salts with acids of diallylamine and diallyldialkylammonium salts; sulfoalkyl acrylates or methacrylates: water-insoluble acrylic or methacrylic acid esters such as methyl acrylate or methacrylate, ethyl acrylate or methacrylate, butyl acrylate or methacrylate, hydroxyethyl acrylate or methacrylate and hydroxypropyl acrylate or methacrylate; acrylamide or methacrylamide derivatives such as acrylamide alkylsulfonates and dialkyl acrylamides or methacrylamides; acrylonitrile; and vinylsulfonic acid or its salts. In general, these polymers having a high degree of polymerization are obtained in the form of a relatively hard gel, but sticking or adhesion to the wall of a polymerization vessel, stirrer, etc. is noticeable and the separation of the polymers from the wall is not easy. Even polymers having a low degree of polymerization tightly adhere to the vessel wall. In practice, the polymers which have once adhered to the vessel wall are very difficult to remove.
In order to eliminate such a problem encountered by the production of water-soluble polymers having a high stickiness or adhesion property, various processes are proposed, e.g. a process wherein removal of the polymers is improved by coating a releasing agent onto the wall surface of a polymerization vessel (as disclosed in Japanese Unexamined Patent Publication No. 41386/1978), a process wherein stainless steel treated, for instance, by electrochemical polishing is used as a vessel material in order to decrease the frictional resistance of the wall surface of a polymerization vessel (as disclosed in Japanese Unexamined Patent Publication No. 10387/1979), and a process wherein removal of the polymers is improved by coating a synthetic resin onto the wall surface of a polymerization vessel (as disclosed in Japanese Unexamined Patent Publication No. 73583/1981).
The above former two processes improve the peelability of the polymers from the wall surface to some extent, but are still insufficient when the stickiness or adhesion property of the produced water-soluble polymers is high.
Generally, coating of the wall surface of a polymerization vessel with synthetic resins is effective in improving the peelability of the sticky polymers from the wall surface. For this purpose, there have been used synthetic resins such as polyethylene, polypropylene, polyester, polyamide, polytetrafluoroethylene, polytrifluorochloroethylene, polyvinyl chloride and epoxy resin. It is known that polytetrafluoroethylene and polytrifluorochloroethylene are particularly useful for improving the peelability.
However, the use of the polymerization vessel coated with the synthetic resins as mentioned above in polymerization of an aqueous solution of the abovementioned vinyl monomers has the disadvantage that conversion of the monomers to the polymers is very low in the vicinity of the boundary between the synthetic resin coating and the produced water-soluble polymer gel, and in an extreme case, the monomers remain unreacted in the vicinity of the boundary, thus resulting in lowering of the polymerization conversion and the yield. Further, in case of toxic monomers such as acrylamide and powerful smelling monomers such as acrylic or methacrylic acid esters, remaining of the unreacted monomers may cause a problem in environmental pollution such as the toxicity for human body in work operation. It has been considered that the low conversion of monomers in the vicinity of the synthetic resin coating is caused by oxygen present on the coating surface. Accordingly, in case of using a polymerization vessel coated with a synthetic resin, it is necessary to select a coating material which does not exert a bad influence on the polymerization behavior.
It is known to be able to prepare the water-soluble polymers by photopolymerization of vinyl monomers with irradiation of ultraviolet rays. It is also known to conduct the photopolymerization by supplying an aqueous solution of the monomers in the form of a thin layer. In that case, the polymerization efficiency is raised by using a polymerization vessel having a surface of a high reflectivity to ultraviolet rays, e.g. a vessel made of mirror finished stainless steel. However, the mirror finished stainless steel is expensive and the use thereof requires a large equipment investment when practicing the photopolymerization on an industrial scale. Further, in case that the produced polymer is sticky, it is not easy to remove the polymer from the surface of the polymerization vessel even if the vessel is made of a mirror finished stainless steel. Since the polymerization of a monomer in the form of a thin layer by irradiation of ultraviolet rays is completed in a very short period of time, it is industrially very important that the sheet of the produced polymer can be easily peeled off from a substrate for supporting the monomer layer.
It is an object of the present invention to provide a process for preparing a water-soluble polymer having a high stickiness or adhesion property in an improved yield without difficulty in recovering the polymer from a polymerization vessel.
A further object of the present invention is to provide a process for preparing a sticky water-soluble polymer by photopolymerization, according to which the polymer can be obtained in a high yield employing an inexpensive polymerization vessel and can be easily recovered from the polymerization system.
These and other objects of the present invention will become apparent from the description hereinafter.